PI HER (Pi Herculis). The 88 formal constellations are accompanied
in the sky by many informal ones, a variety of beloved asterisms,
of which Ursa Major's "Big Dipper" is probably foremost. One of the best
known belongs to Hercules, a northern
quartet of stars forming the "Keystone," which looks very much like
the central keystone in a stone arch that keeps it from falling
down. The southwestern corner of the Keystone is anchored by its
brightest star, Zeta Herculis, while the
northeastern corner is formed by Pi Herculis. An easily visible
third magnitude (3.16), this class K (K3) star shines at us from a
distance of 370 light years, showing it to be quite luminous.
Usually given as a "bright giant" (but sometimes as a supergiant),
Pi Her radiates 1330 solar luminosities into space from its coolish
4110 Kelvin surface. Even at that distance, it is so large that
its angular diameter is easily measurable. Unlike the Sun,
however, large stars have ill-defined and fuzzier "surfaces" such
that the size is dependent on the color at which you measure. In
the visual spectrum, the star measures 0.055 seconds of arc across,
but in the infrared it is larger, 0.061 seconds (the difference
caused by the way that the opacity of the stellar gases change with
color). The resulting actual radii are 55 and 66 times that of the
Sun, the latter corresponding to 0.30
astronomical units, 80 percent the size of Mercury's orbit.
Calculation of the size from temperature and luminosity yields a
larger value of 72 solar radii, showing that some of the numbers
are a bit in error, most likely the allowance for invisible
infrared radiation. Weighing in at 4.5 solar masses, born some 140
million years ago, and most likely fusing helium in its deep core,
Pi Her was once a blue class B5 dwarf. Though you would never know
it with visual observation, Pi Her is a kind of "variable star,"
the star periodically changing its velocity relative to Earth
(found from the spectrum) over a 613 day period. The cause might
be an orbiting substellar companion of at least 27 Jupiter masses
at a distance of 3 AU from Pi itself, "non radial" pulsations (in
which parts of the stellar surface pulse outward while others move
inward), or rotation (that must be less than 1320 days) that brings
spots in and out of the field of view. Given the star's size,
gentle pulsation is probably the best bet.